Explorer links are hardcoded to mainnet throughout the frontend

[Mosas2000]

The frontend still builds explorer URLs with ?chain=mainnet, even in places that should respect the selected network.

Evidence:

• frontend/src/utils/transactions.ts:162-163 returns a mainnet-only tx explorer URL.
• frontend/src/config/network.ts:19-32 already has network-aware explorer helpers, but they are not used everywhere.
• frontend/src/pages/TradePage.tsx and other views also embed mainnet-only explorer links directly.

Impact:

• On testnet, transaction and contract links point to the wrong explorer chain.
• Users cannot easily inspect their transaction or contract pages when they are not on mainnet.

Reproduction:

1. Switch the app to testnet.
2. Open a transaction or contract link from the UI.
3. The link still points to the mainnet explorer and the target is not found.

Priority: Medium

Suggested scope:
Route all explorer links through the existing network-aware helper functions so the chain parameter matches the selected network.